1. Field of the Invention
The present invention relates to an opto-electronic scale reading apparatus used, for example, to determine the magnitude and direction of movement of one member relative to another. Such an apparatus is typically used on coordinate positioning machines such as machine tools or coordinate measuring machines.
2. Description of Related Art
It is known from GB Patent No. 1,504,691 to provide a reflective scale which is scanned by a readhead having an index grating and an analyser grating. The readhead illuminates the scale via the index grating, and a periodic light pattern in the form of interference fringes is formed at the analyser grating as a result. Upon relative movement of the scale and the readhead the fringes move across the analyser grating creating, at a given point on the analyser, a light intensity modulation. In one embodiment of the said apparatus, the analyser grating may be positioned so that its lines extend fractionally obliquely to the interference fringes; this results in the generation of moire fringes. Four photo-detectors behind the analyser grating are offset with respect to a single moire fringe so that four phase-shifted cyclically varying electrical signals are produced, from which the magnitude and direction of said relative movement may be determined.
GB 1,231,029, proposes, in substitution of the analyser grating, a detector array comprising a plurality of photosensitive elements which combine the functions of the analyser grating and the photo-detectors. In order to provide phase-shifted detection signals, two or more of such detector arrays are required.
Both the above constructions suffer from a disadvantage. If, due to contamination of the scale or the readhead with dirt, the intensity of light incident upon one of the detectors or detector arrays differs significantly from that incident upon the other detectors or detector arrays, the phase-shifted signals produced will be significantly modified. This phenomenon is known as selective contamination, and affects the accuracy of the measurement.